Amplifier circuits are widely used in analog signal processing circuits. Amplifiers are commonly constructed with MOS transistors or bipolar transistors. In MOS transistors, the gate, the source, and the drain function as the control, reference, and output terminals, respectively. In bipolar transistors, the base, the emitter, and the collector function as the control, reference, and output terminals, respectively. There are three basic types of amplifiers depending on which terminal is common between the input and output of the amplifier; In MOS technologies, they are the common-source (CS) amplifier, the common-gate (CG) amplifier, and the source-follower (SF). The corresponding amplifier types in bipolar technologies are the common-emitter (CE) amplifier, the common-base (CB) amplifier, and emitter follower (EF). The CS and CE amplifiers are most widely used due to their high voltage gain and high input impedance. The CG and CB amplifiers have better frequency response due to the lack of the Miller effect, but the low input impedance makes them difficult to drive. In addition, the CS, CE, CG, and CB amplifiers have large output resistance on the order of the load resistance, which makes it difficult for them to drive resistive loads. The SF and EF are unable to provide voltage gain, and are primarily employed as buffers.
In many applications, non-inverting amplifiers are required. Although CG and CB amplifiers are non-inverting, their low input impedance typically requires an additional buffer stage such as an SF or EF, thus increasing the power consumption, noise, and area.